An oxide semiconductor such as zinc oxide or indium gallium zinc oxide (IGZO) has the excellent characteristics to serve as an active layer of a semiconductor device, and recently is under development with the aim of application to a TFT, a light-emitting device, a transparent conductive film, and others.
For example, a TFT using an oxide semiconductor is with a high electron mobility and with excellent electrical characteristics compared with the one in which a channel is amorphous silicon (a-Si: H) previously used in a liquid crystal display device. Such a TFT also has advantages of possibly achieving a high mobility even at a low temperature of about room temperature.
On the other hand, the oxide semiconductor is known to be not heat resistant enough, and to cause lattice defects due to the desorption of oxygen, zinc, and others during a heat treatment in a TFT manufacturing process. Such lattice defects reduce the resistance of the oxide semiconductor layer because, electrically, the impurity level becomes low therewith. Therefore, the resulting operation is performed in the normally-ON mode, i.e., in the depletion mode, in which a flow of drain current is provided even with no application of a gate voltage. As a result, with the increase of the defect level, the threshold voltage is reduced, thereby increasing a leakage current.
Previously proposed is to lower the defect level on an interface by configuring, using amorphous aluminum oxide (Al2O3), a gate insulation layer that comes in contact with a channel layer being an oxide semiconductor, for example (as an example, refer to Patent Literature 1.)